A fundamental problem in neurobiology is understanding how the potential for complex behaviors are established. Drosophila melanogaster is an excellent model system to explore this question, given that a robust behavior, mating behavior, is genetically programmed. Drosophila male courtship consists of a series of steps directed towards a female. Females that have not been mated are receptive to male courtship behaviors, whereas those that have been recently mated will reject these advances. There is molecular- genetic evidence demonstrating that the sex hierarchy plays a central role in establishing courtship and mating behaviors in both males and females, through the action of transcription factors encoded by doublesex (dsx) and fruitless (fru). Both dsx and fru produce sex-specific isoforms. Molecular-genetic analyses have demonstrated that both dsx and fru play fundamental roles in the establishment of sex- specific cell fates important for reproductive behaviors. Despite the importance of these transcription factors practically nothing is known about their targets. We will use genomic approaches to identify genes regulated by these transcription factors. We will study the functions of these target genes in the nervous system to gain insight into the genetic specification of behavior. This will provide a foundation for understanding the specification of complex behaviors in other organisms, including humans, for which we have very limited understanding.